The present invention relates to methods for treating otic disorders. In particular the present invention relates to methods for treating otic disorders by local administration of a neurotoxin to a human ear.
The human ear can be divided into an outer ear, a middle ear and an inner ear. The outer ear comprises the auricle (commonly referred to as the ear) and the external acoustic meatus (the auditory canal). The tympanic membrane (commonly called the eardrum) separates the auditory canal from the middle ear (the tympanic cavity). Three small, mobile bones, the incus, malleus and stapes make up an ossicular system which conducts sound through the middle ear to the cochlea. The handle of the malleus is attached to the center of the tympanic membrane. At its opposite end, the malleus is bound to the incus by ligaments, so that movement of the malleus causes the incus to also move. The opposite end of the incus articulates with the stem of the stapes. The faceplate of the stapes rests against the membranous labyrinth in the opening of the oval window, where sound waves are conducted into the inner ear. In the cochlea the sound waves are transduced into coded patterns of impulses transmitted along the afferent cochlear fibers of the vestibulocochlear nerve for analysis in the central auditory pathways of the brain.
The air filled tympanic cavity contains various muscles including the tensor tympani and stapedius muscles. The tensor tympani is a long slender muscle which occupies the bony canal above the osseous pharyngotympanic tube, from which it is separated by a thin bony septum. The tensor tympani muscle receives both motor and proprioceptive innervation. A motor branch derived from the nerve to the medial pterygoid (mandibular division of the V, parasympathetic, trigeminal nerve) passes through the otic (a peripheral, parasympathetic cholinergic) ganglion to the tensor tympani. The stapedius muscle extends from the wall of a conical cavity in the pyramidal eminence, located on the posterior wall of the tympanic cavity. The stapedius is innervated by a branch of the (VII, parasympathetic) facial nerve.
Middle ear structures can be examined endoscopically, as set forth by Karhuketo et al., Endoscopy of the Middle Ear Structures, Acta Otolaryngol (Stockh) 1997; Suppl 529:34-39, the contents of which publication are incorporated herein by reference. There are many diseases of the ear including otis media. Otis media is an inflammation of the middle ear, commonly due to infection, and treatable by antibiotics. Alternate treatments for otis media include analgesics, antipyretics and myringotomy.
Loud noise can cause a muscular reflex to arise which attenuates the effect of excessive loud sound upon the middle and inner ear. Thus, the tensor tympani muscle can contract and pull the handle of the malleus inward while the stapedius muscle contracts and pulls the stapes outward. These two forces oppose each other and result in a high degree of rigidity developing in the ossicular system, thereby greatly reducing (by about 30-40 decibels) conduction of low frequency sounds by the ossicular system. This attenuation reflex can protect the cochlea against the damaging vibrations which would otherwise be induced by loud noise and may also act, mask out low frequency sound in a loud environment, and decrease a person's hearing sensitivity to his own voice.
The inner ear comprises the osseous labyrinth and the contained membranous labyrinth. The osseous labyrinth has three regions, the vestibule, the semicircular canals and the cochlea. The membranous labyrinth can be divided into the vestibular apparatus and the cochlear duct. In the walls of the membranous labyrinth within the vestibular apparatus are five distinct area of specialized sensory epithelium to which the terminal fibers of the vestibular nerve are distributed. Hair cells (epitheliocyti pilosi) in the cochlea are the sensory transducers which collectively detect the amplitude and frequency of sound waves entering the cochlea. The efferent innervation of at least the outer hair cells is cholinergic. Afferent innervation of the hair cells is complex and may involve release of one or more neurotransmitters, including glutamate.
Tinnitus
Tinnitus is a perception of sound which originates in the head. It has been estimated that 36 million Americans have some form of tinnitus and that one third of these have severe tinnitus, that is 12 million Americans hear tinnitus all the time (Vernon. J. A., Tinnitus Treatment and Relief, Allyn & Bacon (1998)). In objective tinnitus, the sound is audible, can be heard upon examination of the patient, and frequently corresponds to respiration. In the more frequent subjective tinnitus the sound cannot be heard by someone other than the patient. Tinnitus can be due to myoclonus of the palatal, tensor tympani and/or stapedius muscles.
Myoclonus is a sudden, involuntary movement caused by a muscle contraction or muscle inhibition and can be classified as physiologic, essential, epileptic and symptomatic myoclonus. Palatal myoclonus is characterized by involuntary movements of the soft palate and pharynx. The rhythmic involvement of the eustachian tube can result in the production of audible clicking sound synchronous with the palatal myoclonus. In palatal myoclonus the patient hears an irregular clicking sound coming form one or both ears The condition is caused by myoclonic contractions in tensor or levator palati muscles or both. The injection of botulinum toxin into the soft palate has been effective to treat palatal myoclonus.
Myoclonus of the middle ear is characterized by abnormal repetitive muscle contractions in the tympanic cavity and can result in subjective or objective tinnitus. Permanent relief has been obtained by sectioning or by lysis of the tendons of the stapedius and tensor tympani muscles.
Inner ear tinnitus has been treated by section of the auditory nerve. Animal studies have shown that drugs, such a aspirin, which are known to cause tinnitus, do so with an increase in activity of the auditory nerve. It has therefore been speculated that a decrease in the endocochlear potential by down regulation of the auditory nerve may alleviate tinnitus.
A particular form of inner ear tinnitus is cochlear synaptic (cochlear nerve dysfunction) tinnitus which is due to functional disturbances of the synapse between cochlear hair cells and afferent dendrites of the auditory nerve. The neurotransmitter at the afferent cochlear synapse is glutamate. The majority of patients with cochlear synaptic tinnitus intravenously infused with the glutamate antagonists glutamic acid diethyl ester and caroverine have noted a tinnitus reduction. Drug therapy for inner ear tinnitus has included benzodiazepine tranquilizers such as valium and Xanas (alprazolam), a powerful anxiolytic drug which has strong addictive properties and can cause personality changes. The local anesthetic lidocaine has been proven to relive tinnitus. Unfortunately, because of serious toxicity, lidocaine must be given intravenously and it's effect lasts for only about 5-30 minutes.
Tensor tympani syndrome is a condition in which increased tension in the tensor tympani muscle produces a fluttering low frequency sound in the ear. In many cases the sound is also felt, as if there is a fluttering insect in the bottom of the ear canal. This is caused by the tympanic membrane being rapidly moved by the fibrillation of this middle ear muscle. Therapy includes section of the tendon of the tensor tympani muscle behind the neck of the malleus.
Tinnitus resulting from Meniere's disease can be treated by sectioning the vestibular nerve. Auditory nerve section has been used as a means of treating intractable tinnitus, often with the condition worsening because the tinnitus was not due to a cochlea disorder, prior to irreversible ablative surgery, after which no residual hearing will remain in the ear operated upon.
Botulinum Toxin
The anaerobic, gram positive bacterium Clostridium botulinum produces a potent polypeptide neurotoxin, botulinum toxin, which causes a neuroparalytic illness in humans and animals referred to as botulism. The spores of Clostridium botulinum are found in soil and can grow in improperly sterilized and sealed food containers of home based canneries, which are the cause of many of the cases of botulism. The effects of botulism typically appear 18 to 36 hours after eating the foodstuffs infected with a Clostridium botulinum culture or spores. The botulinum toxin can apparently pass unattenuated through the lining of the gut and attack peripheral motor neurons. Symptoms of botulinum toxin intoxication can progress from difficulty walking, swallowing, and speaking to paralysis of the respiratory muscles and death.
Botulinum toxin type A is the most lethal natural biological agent known to man. About 50 picograms of botulinum toxin (purified neurotoxin complex) type A.sup.1 is a LD.sub.50 in mice. One unit (U) of botulinum toxin is defined as the LD.sub.50 upon intraperitoneal injection into female Swiss Webster mice weighing 18-20 grams each. Seven immunologically distinct botulinum neurotoxins have been characterized, these being respectively botulinum neurotoxin serotypes A, B, C.sub.1, D, E, F and G each of which is distinguished by neutralization with type-specific antibodies. The different serotypes of botulinum toxin vary in the animal species that they affect and in the severity and duration of the paralysis they evoke. For example, it has been determined that botulinum toxin type A is 500 times more potent, as measured by the rate of paralysis produced in the rat, than is botulinum toxin type B. Additionally, botulinum toxin type B has been determined to be non-toxic in primates at a dose of 480 U/kg which is about 12 times the primate LD.sub.50 for botulinum toxin type A. Botulinum toxin apparently binds with high affinity to cholinergic motor neurons, is translocated into the neuron and blocks the release of acetylcholine.
Botulinum toxins have been used in clinical settings for the treatment of neuromuscular disorders characterized by hyperactive skeletal muscles. Botulinum toxin type A has been approved by the U.S. Food and Drug Administration for the treatment of blepharospasm, strabismus and hemifacial spasm. Non type A botulinum toxin serotypes apparently have a lower potency and/or a shorter duration of activity as compared to botulinum toxin type A. Clinical effects of peripheral intramuscular botulinum toxin type A are usually seen within one week of injection. The typical duration of symptomatic relief from a single intramuscular injection of botulinum toxin type A averages about three months.
Although all the botulinum toxins serotypes apparently inhibit release of the neurotransmitter acetylcholine at the neuromuscular junction, they do so by affecting different neurosecretory proteins and/or cleaving these proteins at different sites. For example, botulinum types A and E both cleave the 25 kiloDalton (kD) synaptosomal associated protein (SNAP-25), but they target different amino acid sequences within this protein. Botulinum toxin types B, D, F and G act on vesicle-associated protein (VAMP, also called synaptobrevin), with each serotype cleaving the protein at a different site. Finally, botulinum toxin type C.sub.1 has been shown to cleave both syntaxin and SNAP-25. These differences in mechanism of action may affect the relative potency and/or duration of action of the various botulinum toxin serotypes.
The molecular weight of the botulinum toxin protein molecule, for all seven of the known botulinum toxin serotypes, is about 150 kD. Interestingly, the botulinum toxins are released by Clostridial bacterium as complexes comprising the 150 kD botulinum toxin protein molecule along with associated non-toxin proteins. Thus, the botulinum toxin type A complex can be produced by Clostridial bacterium as 900 kD, 500 kD and 300 kD forms. Botulinum toxin types B and C, is apparently produced as only a 500 kD complex. Botulinum toxin type D is produced as both 300 kD and 500 kD complexes. Finally, botulinum toxin types E and F are produced as only approximately 300 kD complexes. The complexes (i.e. molecular weight greater than about 150 kD) are believed to contain a non-toxin hemaglutinin protein and a non-toxin and non-toxic nonhemaglutinin protein. These two non-toxin proteins (which along with the botulinum toxin molecule comprise the relevant neurotoxin complex) may act to provide stability against denaturation to the botulinum toxin molecule and protection against digestive acids when toxin is ingested. Additionally, it is possible that the larger (greater than about 150 kD molecular weight) botulinum toxin complexes may result in a slower rate of diffusion of the botulinum toxin away from a site of intramuscular injection of a botulinum toxin complex.
In vitro studies have indicated that botulinum toxin inhibits potassium cation induced release of both acetylcholine and norepinephrine from primary cell cultures of brainstem tissue. Additionally, it has been reported that botulinum toxin inhibits the evoked release of both glycine and glutamate in primary cultures of spinal cord neurons and that in brain synaptosome preparations botulinum toxin inhibits the release of each of the neurotransmitters acetylcholine, dopamine, norepinephrine, CGRP and glutamate.
Botulinum toxin type A can be obtained by establishing and growing cultures of Clostridium botulinum in a fermenter and then harvesting and purifying the fermented mixture in accordance with known procedures. All the botulinum toxin serotypes are initially synthesized as inactive single chain proteins which must be cleaved or nicked by proteases to become neuroactive. The bacterial strains that make botulinum toxin serotypes A and G possess endogenous proteases and serotypes A and G can therefore be recovered from bacterial cultures in predominantly their active form. In contrast, botulinum toxin serotypes C.sub.1, D and E are synthesized by nonproteolytic strains and are therefore typically unactivated when recovered from culture. Serotypes B and F are produced by both proteolytic and nonproteolytic strains and therefore can be recovered in either the active or inactive form. However, even the proteolytic strains that produce, for example, the botulinum toxin type B serotype only cleave a portion of the toxin produced. The exact proportion of nicked to unnicked molecules depends on the length of incubation and the temperature of the culture. Therefore, a certain percentage of any preparation of, for example, the botulinum toxin type B toxin is likely to be inactive, possibly accounting for the known significantly lower potency of botulinum toxin type B as compared to botulinum toxin type A. The presence of inactive botulinum toxin molecules in a clinical preparation will contribute to the overall protein load of the preparation, which has been linked to increased antigenicity, without contributing to its clinical efficacy. Additionally, it is known that botulinum toxin type B has, upon intramuscular injection, a shorter duration of activity and is also less potent than botulinum toxin type A at the same dose level.
It has been reported that botulinum toxin type A has been used in clinical settings as follows:
(1) about 75-125 units of BOTOX.RTM. per intramuscular injection (multiple muscles) to treat cervical dystonia; PA1 (2) 5-10 units of BOTOX.RTM. per intramuscular injection to treat glabellar lines (brow furrows) (5 units injected intramuscularly into the procerus muscle and 10 units injected intramuscularly into each corrugator supercilii muscle); PA1 (3) about 30-80 units of BOTOX.RTM. to treat constipation by intrasphincter injection of the puborectalis muscle; PA1 (4) about 1-5 units per muscle of intramuscularly injected BOTOX.RTM. to treat blepharospasm by injecting the lateral pre-tarsal orbicularis oculi muscle of the upper lid and the lateral pre-tarsal orbicularis oculi of the lower lid. PA1 (5) to treat strabismus, extraocular muscles have been injected intramuscularly with between about 1-5 units of BOTOX.RTM., the amount injected varying based upon both the size of the muscle to be injected and the extent of muscle paralysis desired (i.e. amount of diopter correction desired). PA1 (6) to treat upper limb spasticity following stroke by intramuscular injections of BOTOX.RTM. into five different upper limb flexor muscles, as follows:
(a) flexor digitorum profundus: 7.5 U to 30 U PA2 (b) flexor digitorum sublimus: 7.5 U to 30 U PA2 (c) flexor carpi ulnaris: 10 U to 40 U PA2 (d) flexor carpi radialis: 15 U to 60 U PA2 (e) biceps brachii: 50 U to 200 U. Each of the five indicated muscles has been injected at the same treatment session, so that the patient receives from 90 U to 360 U of upper limb flexor muscle BOTOX.RTM. by intramuscular injection at each treatment session.
What is needed therefore is an effective, non-surgical ablation, non-radiotherapy therapeutic method for treating otic disorders, including middle ear tinnitus, inner ear tinnitus and myoclonic ear muscle tinnitus.